Electron microcope whith integrated detector(s)

ABSTRACT

An electron microscope including a vacuum chamber for containing a specimen to be analyzed, an optics column, including an electron source and a final probe forming lens, for focusing electrons emitted from the electron source, a specimen stage positioned in the vacuum chamber under the probe forming lens for holding the specimen, and an x-ray detector positioned within the vacuum chamber. The x-ray detector includes an x-ray sensitive solid-state sensor and a mechanical support system for supporting and positioning the detector, including the sensor, within the vacuum chamber. The entirety of the mechanical support system is contained within the vacuum chamber. Multiple detectors of different types may be supported within the vacuum chamber on the mechanical support system. The mechanical support system may also include at least one thermoelectric cooler element for thermo-electrically cooling the x-ray sensors.

CROSS REFERENCE

This application depends upon U.S. Provisional Application No.61/216,290, filed 15 May 2009, entitled ELECTRON MICROSCOPE WITHINTEGRATED DETECTOR(S), the entirety of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The invention pertains to the integration of semiconductor x-rayradiation sensors within a Scanning Electron Microscope (SEM) or similaranalytical electron-beam instrument, optionally in conjunction with abackscattered electron sensor. The invention pertains to novel methodsof configuring both the detection elements and the microscope so as toachieve improvements in performance and economies of construction, aswell as other benefits.

The installation of a solid-state Energy Dispersive X-ray (EDX) detectoronto an electron microscope was first reported by Fitzgerald, Keil, andHeinrich in 1968. The type of detector described was a lithium-driftedsilicon (Si(Li)) diode that was introduced through the port of anelectron probe micro analyzer (EPMA). This kind of detector was sooncommercialized and units of this same general type have been installedon many kinds of Electron Microscope (EM), notably including theElectron Probe Micro-Analyzer (EPMA), Scanning Electron Microscope(SEM), Transmission Electron Microscope (TEM), and Scanning TransmissionElectron Microscope (STEM). Though the technology has been greatlyrefined over the years, the EDX units themselves have retained certainsignificant characteristics of the earliest models.

FIG. 1 is a schematic representation of a conventional Si(Li) EDXdetector unit [60]. The active sensor element [1] is the Si(Li) diodeand installed in close proximity is the sensor's field effect transistor(FET) [2] which amplifies the current pulse produced by the sensorelement. Both the sensor element and the FET are mounted on the anteriorend of a thermally conductive rod [3] that is in thermal contact at itsposterior end with a reservoir of liquid nitrogen [4] contained in aninsulated container called a dewar. The thermally conductive rod (“coldfinger”), the sensor diode, and the FET are all enclosed in a tubularhermetic envelope [5] that is maintained at a high vacuum, both tominimize thermal transfer, and to maintain the sensor in a cleancondition. The anterior end of the tubular hermetic envelope is closedwith a thin window [6] that admits x-rays to the front face of thesensor but maintains the interior vacuum. The liquid nitrogen (LN)reservoir [4] is an essential aspect of the design of the conventionalSi(Li) detector since it is required that the sensor element and FET bemaintained at a cryogenic temperature if acceptable energy resolution isto be achieved. A collection of circuitry conventionally known as a“preamplifier” [61] receives and further amplifies the signal receivedfrom the sensor FET [2] as well as providing additional electronicfunctions in support of the sensor operation. In order to implement acomplete EDX system, the detector unit of FIG. 1 would be supplementedwith additional electronic components. Such components conventionallyprovide for necessary electrical power, digitization of the waveformreceived from the preamplifier [61], collection and display of theresulting x-ray spectrum, and spectrum analysis functions. Suchadditional support components are known in the art and are not the focusof the present invention.

FIG. 2 schematically illustrates the manner in which a conventionalSi(Li) EDX detector unit [60] is installed on a conventional SEM or EPMA(which are physically similar instruments). The EDX detector unit ismounted on a flange [7] that is bolted onto one of the several portsusually provided for such use on the specimen chamber [8] of theelectron microscope. The sensor end of the EDX detector [60] is thusbrought into close proximity to the specimen [9] (positioned on specimenstage [9′]), to intercept the x-rays emitted at the point of impact ofthe focused electron beam [10]. The electron beam is produced by theelectron optics “column” [11]. A backscattered electron (BSE) detector[62] is commonly mounted beneath the final focusing lens [11′] of theelectron column [11]. The specimen chamber [8] is shown with one sideopen for purposes of this illustration, but in use, the specimen chambermust be sealed and evacuated (and is thus interchangeably known as the“vacuum chamber”). Thus, the detector tube [5] of the detector unit [60]must be vacuum-sealed at the flange [7] where it passes through thevacuum chamber wall. Customarily this involves a radial o-ring seal tothe exterior of the tube, and typically this is designed as a slidingseal so that the insertion of the detector relative to the specimen maybe varied. Thus the cylindrical tube [5], the flange [7], and the “coldfinger” [3] function in a complementary fashion to support the sensorelement [1] in its desired relation to the specimen. This support systemmay be configured in a variety of ways, but it is physically anchored bythe flange [7] which is mounted to the exterior of the vacuum chamber[8].

The above figures and explanations describe, in a very general manner,the standard design of the Si(Li) EDX detector and the manner in whichit is installed on an electron microscope of the SEM/EPMA type whereinthe specimen stage [9′] carrying a bulk specimen [9] is located beneaththe final probe-forming lens [11′] which constitutes the bottom elementof the electron optics column [11]. Si(Li) detectors of similar designhave also been employed with electron microscopes of the STEM/TEM typewherein a small electron-transmissive specimen is supported in the gapof an objective lens. Though certain specialized considerations maypertain to EDX detectors employed for STEM/TEM types of instrument,those detectors too have also generally conformed in the historicalpractice to the principles of construction illustrated in FIG. 1.

Pertaining to the interface of an EDX detector to an electron microscopeof the SEM/EPMA type (which is the field of the subject invention),there are several important considerations affecting the performance ofthe detector:

-   -   (a) A large detection solid angle is desirable in order to        maximize the number of detected x-rays (and thus the statistical        precision) that can be achieved for a given beam current and        measurement time.    -   (b) A high take-off angle is desirable in order to minimize        absorption effects as the excited x-rays exit from a point of        origin below the surface of the specimen.    -   (c) Optimal collimation of the detected x-rays is facilitated by        pointing the detector coaxially at the intended beam impact        point on the specimen. Such collimation ensures that only the        uniformly responsive area of the sensor is employed for        detection and that x-rays originating from scattered electrons        are excluded.

Thus, in an “ideal” situation, the sensor element would be located veryclose to the specimen, with its axis in line with the intended beamimpact point, and inclined at a high take-off angle. However, it is alsodesirable for the focusing lens of the microscope to be in closeproximity to the specimen and, for the many applications in which abackscattered electron detector is required, that the BSE detector'sview of the specimen should not be obscured. Thus, the space under thefocusing lens is both small and crowded and this restricts theattainment of ideal detector geometry. Further, the physical arrangementof the specimen chamber, such as the presence of access doors andauxiliary ports, will play a large role in restricting where and howdetectors may be placed.

It can be appreciated that the above considerations, when coupled withthe over-riding necessity of thermally coupling the Si(Li) sensor to anexternal cryogenic cooler, have shaped the evolution of the traditionalEDX detector unit into the familiar tube-mounted configurationillustrated in FIG. 1 (which has sometimes been descriptively referredto as a “sensor on a stick”). In turn, the standardization on this kindof tube-mounted configuration has also affected the design of electronmicroscopes which, by necessity, are specifically configured formounting of such detectors. Since x-ray detectors have historically beendesigned and manufactured by one group of suppliers and electronmicroscopes by another, departure from this familiar model has not beenattractive to either group.

Within the past decade, the technology of EDX detectors has beenradically altered by the introduction of highly capable x-ray sensorsthat do not require cryogenic cooling. The principal current embodimentof this type of detector is the so-called “silicon drift detector” (SDD)whose operation is described in the scientific literature. These devicesachieve spectroscopic performance generally superior to that of theSi(Li) detector, but at temperatures that can be conveniently achievedwith a small thermoelectric cooler (TEC) based on the Peltier principle.

FIG. 3 illustrates the basic structure of a packaged SDD module as maycurrently be purchased from a manufacturer (PNDetector GmbH of Munich,Germany) of such devices. Within such a module, the SDD sensor element[12] is a small planar “chip” manufactured by semiconductor processes.The SDD sensor chip is mounted on the cold face of a small TEC device[13] which is attached to a housing base [14]. A thermally conductivestud [15] is in thermal contact with the warm side of the TEC device andserves as the external “sink” attachment by which heat generated by theTEC is removed from the module. A collimator plate [16] is typicallymounted in front of the SDD sensor element [12] so as to permit x-raysto strike only the intended active area. A housing cap [17] is sealed tothe housing base [14]. The front of the housing cap [17] is closed witha thin window [18] which permits x-rays to enter and strike the SDDsensor element [12]. The housing base [14] is provided with an array ofelectrical connection pins [19] through which power and control signalsmay be provided to the sensor and the TEC, and through which outputsignals may be extracted (connections of these pins to the internalelements are omitted from the figure). One of the signals available viathe output pins is the front-face temperature of the TEC module [13],which permits external circuitry to regulate a constant operatingtemperature for the sensor element [12] (the temperature sensor requiredfor this regulation is not shown in the figure). The housing base [14]is typically configured according to the “TO-X” convention that has beenused within the electronics industry for mounting various types ofdevices and sensors (e.g., the TO-8 case). The entire module ishermetically sealed and may be evacuated or filled with an inert gas ata reduced pressure.

The FET which amplifies the weak current pulse of the sensor remains akey component of the SDD device and must be located in close proximityto its anode electrode, just as for the Si(Li) technology. However,rather than implementing the FET [2] as a discrete element asillustrated in FIG. 1, one manufacturer of SDD sensors integrates theFET circuitry into the same semiconductor die [12] as the sensor itself,and this is the type of device illustrated in FIG. 3. Othermanufacturers mount a discrete FET in close proximity to the anode.Regardless of whether the FET is integrated or discrete, however, it isunderstood that a FET is associated with the SDD sensor.

The use of the kind of packaged SDD module here illustrated is notrequired for the implementation of an x-ray detector based on SDDtechnology. Indeed, it is believed that at least one manufacturer of EDXdetector units places the unpackaged elements of the sensor moduledirectly on the end of a cold finger and encloses it in an evacuatedtube, thus closely mirroring the conventional construction of the Si(Li)detector illustrated in FIG. 1. There is both specialized art andspecialized equipment involved in mounting and packaging the SDD sensorand, to date, the majority of x-ray detector manufacturers have chosento purchase pre-packaged modules from one of the specialized suppliersof such modules. Thus, for the remainder of this discussion the use of apackaged SDD module, such as illustrated in FIG. 3 will be assumed.However, it will be apparent that the same principles taught in thesubject invention could be equally applied to the construction of adetector assembled from unpackaged components.

In order for the packaged SDD module of FIG. 3 to be made into afunctional x-ray detector unit, it must be provided with several things:

-   -   1) Power supplies and control signals to operate the SDD sensor        and its incorporated FET.    -   2) Preamplifier circuitry to amplify the weak signal from the        sensor FET so as to produce a robust waveform that can be        quantified.    -   3) Temperature control electronics that actively regulates the        power provided to the internal TEC element [13] so as to        maintain the desired operating temperature of the SDD sensor        element [12].    -   4) Mechanical elements to support the module in the required        proximity to the specimen.    -   5) A thermal path to a thermal sink of sufficiently low        temperature to which the thermal stud [15] may discharge the        heat generated by the module.

To date, commercial SDD detectors have accomplished these provisions ina package that closely emulates the format of the traditional Si(Li)detector. FIG. 4 depicts schematically a conventional SDD detectoremploying a packaged SDD module. The packaged sensor module [20] isattached to the anterior end of a suitable thermally conductive rod orpipe [21], whose posterior end terminates with a heat dissipatingmechanism that is incorporated in the body of the detector which isexterior to the vacuum chamber. A housing tube [71] encloses thethermally conductive rod [21] and provides the sealing surface by whichthe detector snout maintains a vacuum-tight connection as it passesthrough the mounting flange attached to a microscope (e.g., flange [7]of FIG. 2). A thermal insulating element [72] provides thermal isolationof the housing tube [71] from the thermally conductive rod [21] whileproviding mechanical support. The heat dissipating mechanismincorporated in the body of the detector exterior to the vacuum chambermight take various forms (for example, in one known case, a chilledwater cooler has been employed). In the specific case illustrated, thethermally conductive rod [21] is terminated with a plate [73] whichabuts the “cold” side of a second TEC element [22]. The “warm” side ofthe second TEC element [22] is in contact with an air-cooled heat sink[23] by which the heat generated by the second TEC element is dissipatedto the environment. This heat sink, equipped with fins or similarstructures to facilitate convection, is typically integrated into theexterior case of the detector unit. Contained in this case is acollection of electronic circuitry elements [24] which operate thedetector and the second TEC element and provide the “preamplifier”function. The above is, of course, a highly generalized illustration ofthe internal components of a complete SDD detector and is subject tovariation in detail, but nonetheless generally conforms to precedentsestablished by the conventional Si(Li) detector:

-   -   Firstly, it is apparent that this conventional design adheres to        the precedent of the LN-cooled Si(Li) detector, in that the        sensor is brought into proximity with the specimen by inserting        it into the specimen chamber at the end of a straight tubular        element whose form and function is equivalent to that employed        by the conventional Si(Li) detector.    -   Secondly, the thermal circuit of the conventional SDD unit        continues to utilize the same “cold finger” strategy employed by        the thermal circuit of the Si(Li) detector. Though its role in        the SDD is to act as the heat-extraction conduit required by the        embedded TEC cooler [13], rather than to directly cool the        sensor, as for the Si(Li), in both cases the cold finger        provides the thermal connection to an external heat-dissipating        sink which is isolated from the specimen chamber.    -   Finally, like the Si(Li) detector, the conventional SDD detector        is a modular unit which positions the sensor by means of a        support system, including a tubular enclosure, which is again        anchored externally by a flange attached to the vacuum chamber        of an electron microscope.

In short, in transitioning from Si(Li) sensor technology to SDD sensortechnology, detector manufacturers have effectively retained theconventional detector design with the substitution of: (a) an SDD modulefor the Si(Li) diode and (b) typically a TEC module and air-cooled heatsink substituted for the LN dewar. This has been a rather logicalmigration path for both EM manufacturers and EDX detector manufacturerssince it retains the conventional format of the Si(Li) detector and thishas a number of commercial benefits in terms of compatibility with pastand present microscope designs. However, this conventional practice doesnot take advantage of the opportunities to effect improvements inperformance and cost reduction that are created by the alteredconstraints associated with an x-ray detector that does not requirecryogenic cooling.

PRIOR ART

The above background has illustrated the general principles andpractices of the conventional EDX detector of both Si(Li) and SDD types,as well as the practices by which such detectors are installed inelectron microscopes of conventional design. Prior practice has soughtto optimize these principles and practices in various ways which willnow be described.

Large Area Detectors

Collection efficiency is one of the most important characteristics of anEDX detector since it dictates the speed and precision of measurements.An obvious strategy for improving collection efficiency is to increasethe active area of the sensor, thereby increasing the solid angle. Forthe Si(Li) detector, this strategy is limited by the direct relationshipbetween detector noise and active area, meaning that resolution rapidlydegrades as detector size increases. Consequently, Si(Li) detectors withactive areas of greater than 30 mm² have rarely been used in EMapplications. The technology of the SDD detector, on the other hand,largely decouples the detector area from the noise characteristic, sothat SDD performance is much less affected by the active area of thistype of sensor. Consequently, 80 mm² SDD detector units are now marketedfor electron microscope applications and suitable packaged sensormodules of area up to 100 mm² are commercially offered. Though suchincreases in detector size are certainly beneficial in certain respects,they also have drawbacks, and especially within the context of theconventional tubular-mount EDX design. The diameter of the detector tubemust accommodate the size of the sensor device with allowance formounting and connections, and this increased tube diameter limits theoptimal placement of the detector. Because of the necessity of avoidinginterferences with the final focusing lens, the BSE detector, and thespecimen, a larger diameter detector must be somewhat retracted and/oroperated at a lower takeoff angle. To avoid these consequences, it wouldbe necessary to increase the working distance of the specimen to theface of the final focusing lens, which is in turn detrimental to theoptical performance of the microscope. Thus, it will be appreciated thatcompromises and diminishing returns ensue when increased detector solidangle is accompanied by an increase in detector tube diameter.

In addition to such geometrical considerations, increasing the detectionefficiency by increasing detector area creates other problems when thereis a large flux of x-rays impinging on the detector. An SDD detectorequipped with modern electronics can detect x-rays at rates upwards of100,000 events/second. However, at such high rates there is also anincreased probability of “summing events” in which two different x-rayemissions reach the detector so close together in time that they cannotbe distinguished as separate events. This effect leads to “sum peaks”and other artifacts in the measured x-ray spectrum, and this in turncreates problems relative to accurate analysis. Thus, a large-areadetector operated in close proximity to a specimen may be advantageousfor analytical circumstances where the x-ray rate is low, but isproblematic when the rate is high. Of course, the x-ray detection ratecan always be reduced by either reducing the electron beam intensity,thereby reducing the number of x-rays produced, or by withdrawing thedetector so as to reduce the solid angle (many detectors are mounted onslides for just this reason). However, exercising either of thesestrategies tends to defeat the point of a large-area detector, andneither of these strategies is optimal when the specimen has bothhigh-emission and low-emission regions.

Finally, there are other, more subtle issues associated withvery-large-area x-ray sensors. One is that the large sensitive areamakes it more difficult to collimate the x-ray path to accept x-raysemitted from the point of beam impact and exclude those produced byscattered electrons striking elsewhere. Also, although the energyresolution of SDD detectors does not degrade as significantly as for theSi(Li) detector as the sensor area is increased, there is still someloss of resolution in the current generations of SDD devices. Andfinally, large-area SDD sensors of spectroscopic quality aresubstantially more expensive at present than smaller devices.

Thus, the strategy of increasing x-ray detection efficiency byincreasing the area of the sensor element is a viable option up to apoint, and then other factors increasingly diminish its attractiveness.

Multiple Detectors

An alternative manner of increasing the x-ray detection efficiency hasbeen to employ multiple detectors. This has sometimes been employed withSi(Li) detectors for specialized applications where x-ray collectionspeed is very important, the microscope is equipped with appropriatemounting ports, and the high cost is warranted. Since this strategyutilizes an independent set of counting electronics for each sensorelement, this strategy for multiplying the effective detection solidangle can be accomplished without exacerbating the problem of summingevents.

There is also growing recognition that multiple detectors can not onlyprovide greater sensitivity, but can facilitate the analysis of non-flatspecimens (such as particulate specimens or fracture surfaces) wheretopographic effects (such as shadowing) can produce misleading resultsvia a single detector. Further, the elimination of the bulkyliquid-nitrogen dewar has made mounting multiple SDD detectors morefeasible. For the above reasons, there has been increasing interestwithin the industry for employing multiple EDX detectors and certain SEMunits have been designed with this provision in mind.

Multiple-Element Sensor Arrays

A strategy that can be employed to gain the benefits of multipledetectors without a proliferation of ports on the electron microscope isto incorporate multiple sensor elements in a single detector housing.This is an attractive option for SDD sensors because they are fabricatedvia semiconductor lithography technology. Thus it is relatively simpleto manufacture SDD arrays that incorporate multiple sensor elements inthe same die, and such multi-element sensor arrays are commerciallyavailable for integration into an x-ray detector unit. A certaincommercial detector unit mounts an array of four 10 mm² sensor elementsin a single detector tube, with the array perpendicular to the axis ofthe tube, and provides each sensor with its own electronics processingchannel. In this manner, the single detector provides an equivalentdetection solid angle of a 40 mm² sensor, but can sustain much highercounting rates than a mono-element detector of this size due to theparallel processing electronics. However, because of the intrinsic sizeof such an array, this kind of detector requires an especially largemounting tube and thus again sacrifices some sensitivity due to setbackfrom the specimen. Thus, this kind of detector is best suited forapplications (such as EPMA analysis) where high beam currents areemployed, and high count rate is more important than high-sensitivity.

Another commercial variant of the multi-element sensor array conceptarranges the sensor elements around a central orifice. When implementedas a detector for electron microscopes, the sensor array is incorporatedin a flat housing with a central passage for the beam. The housing isinserted under the final focusing lens of an electron microscope,immediately above the specimen so that the effect is that of foursensors arrayed around the axis of the beam. This arrangement can resultin quite favorable detection solid angle and thus provides highsensitivity, as well as the benefits of a symmetric detector array foranalyzing rough specimens. However, this sensor occupies the positionnormally reserved for a BSE detector, and it is thus not suitable forthe many applications where a high-quality BSE signal is required. It isalso notable that the single current instance of a commercial detectorunit of this type attaches the array to the end of a tube that isinserted from a side port in the specimen chamber, and thus propagates avariant of the conventional tubular port-mounted configuration.

It is apparent that the multi-element SDD device can be a very effectivemeans for achieving higher counting rates. Such detectors combinecertain of the advantages of both the large area detector and multipledetectors, but they are presently rather costly devices whose specialinterfacing requirements present challenges for installation in somemicroscopes. Architecturally, their embodiments for SEM-type instrumentshave been implemented as straight-forward extensions of conventionalmodular tube-mount detector geometries.

Entry Window and Electron Trap Considerations

The x-ray window [6] or [18] hermetically seals the detector whilepermitting the entry of x-rays. Two different basic window types areemployed: Beryllium windows are relatively inexpensive and rugged, butare opaque to low-energy x-rays. Consequently, beryllium window (BeW)detectors are not generally useful for measuring elements lower thanfluorine in the periodic table. Ultra-thin windows (UTW) are fabricatedfrom a thin material which transmits low-energy x-rays, thus permittingdetection of elements as low as beryllium. However, UTW detectorsrequire a magnetic electron trap to prevent energetic scatteredelectrons from penetrating the x-ray window and striking the sensor,thus corrupting the measured x-ray spectrum. Because the strength of thepermanent magnets employed in such a trap is limited by available magnettechnology, an increase in the aperture area of such a trap necessarilyresults in weakened deflection field strength while the amount ofrequired deflection is simultaneously increased. Thus the only recourseis to increase the depth of the trap. Novel constructionsnotwithstanding, a UTW detector with a large-area sensor must thereforebe set back from the specimen further than a smaller-area sensor due tothe increased depth of the electron trap. This consideration isapplicable for both single element sensors and multi-element sensorarrays and can represent a significant limitation to attainable solidangle when light-element detection is required.

Tilted-Sensor Detectors

The tilted-sensor detector was encountered rather commonly in the earlydays of EDX technology because electron microscopes of the day had notbeen designed for installation of high-takeoff-angle detectors. Many ofthe older SEM instruments had flat-bottomed optics (rather than thetruncated-cone style illustrated in FIG. 2) and access ports werelocated parallel to the bottom of the pole piece. To accommodate such ageometry, the Si(Li) diode element was mounted at an angle within thedetector tube such that the tube would protrude horizontally under thepole piece, but the sensor axis would be inclined relative to thespecimen. This strategy works reasonably well for small-area detectors,but becomes cumbersome to implement as the sensor size is increased. Ithas thus far had limited use for an SDD type of detector. The relativeunpopularity of this type of mount can be largely attributed to thedifficulty in installing a large-area sensor or sensor module in anon-axial orientation within the conventional tube-type mount.

Summary of Conventional X-Ray Detector Art

It will be seen from the above discussion of prior art that a number ofimaginative approaches have been applied to the challenge of configuringEDX detectors in order to optimize performance. The introduction of SDDtechnology has provided a new variable in terms of larger-area sensorsand multiple-sensor units. However, through the 40 years of EDXdevelopment, and continuing through the present, there are certainthings that have not varied at all. These include:

-   -   1. The “sensor on a stick” format whereby the sensor is mounted        at the end of a straight rod or tube that extends into the        specimen chamber.    -   2. The modular port-mounted arrangement, whereby the detector is        a discrete entity from the microscope and is supported within        the specimen chamber via attachment to an exterior port.    -   3. The cold finger approach in which the sensor is cooled via a        copper bar, heat pipe, or similar thermally conductive rod that        extends through the vacuum envelope of the microscope.    -   4. The external cold source approach in which a “sink”, distinct        from the microscope itself and operating at ambient or        lower-than-ambient temperature, is incorporated in the detector        body located external to the microscope's specimen chamber.    -   5. The external electronics approach, in which all operational        electronics required to operate the detector (other than the        sensor's associated FET) is located exterior to the vacuum        chamber.    -   6. The modularity of the x-ray detector which implements each        detector unit as a separate entity, distinct from the microscope        and from other detectors that are also used in the electron        microscope.

Many of the above conventions derived directly or indirectly frompractical considerations associated with the requirement for an LNreservoir for cooling of the Si(Li) sensor—a constraint that is nolonger relevant for SDD sensor technology. At the same time, theavailability of the critical SDD sensor technology as packaged moduleshas made it more practical for an electron microscope manufacturer toutilize this technology in unconventional ways. However, long-standingconventions regarding x-ray detector and electron microscopeconfigurations, as well as commercial factors related to the partitionof the industry into detector manufacturers and microscope manufacturershave acted to inhibit innovations which would depart from long-acceptedconventions. In this context, the present inventors have benefitted fromthe unique perspective accruing to decades of involvement in both x-raydetector development and electron microscope development and have beenable to perceive unique opportunity in the integrative approach heredisclosed.

SUMMARY OF THE INVENTION

The present invention teaches a new approach to the design of anelectron microscope, in which one or more energy-dispersive sensors areintegrated directly into the structure of the microscope, therebyrealizing a number of important benefits. The methods taught aredirectly applicable to SDD technology, but are also applicable to othertypes of solid-state x-ray sensor (such as the PIN diode, or CCDdevices) which do not require liquid nitrogen cooling. The essence ofthese innovations is to abandon the “sensor on a stick” configurationwhich characterizes past and present x-ray detectors of conventionaltube-mount design. Rather, it is shown how the x-ray detector functionmay be advantageously integrated into the structure of the EM itself. Itwill also be shown how certain of the innovative design elementsdisclosed here may also be advantageously employed in the context of aport-mounted detector of novel design.

The type of electron microscope towards which the subject innovationsare specifically directed is one of the SEM/EPMA type wherein thespecimen stage is located exterior to and below the final focusing lens.However, certain beneficial aspects of the innovations herein taught mayalso be applicable to electron microscopes of other configurations.

The essence of the subject invention is to depart from the traditional“sensor on a stick” (tube mount) geometry of conventional x-raydetectors by incorporating the sensor element(s) directly into thespecimen chamber of an electron microscope of the SEM/EPMA type. Thus,rather than being an independent module inserted from the exterior, thesubject invention implements the x-ray detector as an internal componentof the microscope itself. This new approach has several advantageouscharacteristics:

-   -   1. It permits more optimal placement of the sensor(s) relative        to the specimen since the sensor need not be located at the end        of a straight rod or tube inserted from a mounting surface        exterior to the microscope.    -   2. It permits more optimal design of the microscope, since the        geometry is not constrained by the need to provide a direct line        between the desired location of the sensor and an external port.    -   3. By eliminating much of the spatial “overhead” associated with        tube mounting, it permits the sensor element to be more closely        positioned to the specimen, thereby allowing the benefits of a        larger sensor to be achieved with a smaller and less costly one.    -   4. It more readily and economically supports a multiplicity of        detectors since separate ports need not be provided for each,        and certain capabilities (such as thermal circuits and        electronic connections) can be shared.    -   5. It facilitates more flexible and optimal integration of x-ray        detectors with other detector types (e.g., BSE detector).    -   6. It permits more efficient approaches to thermal management of        TEC-cooled sensors to be employed.    -   7. It facilitates the location of critical amplification and        support electronics in close proximity to the sensor(s) to        minimize noise pickup and reduce lead capacitance.

The electron microscope of the present invention incorporates twoprimary features, one being the physical integration of the solid statex-ray detector and the second pertaining to the thermal management ofthe integrated x-ray detector. With reference to the first feature, theelectron microscope of the present invention is comprised of a vacuumchamber for containing a specimen to be analyzed, an optics column,including an electron source and a final probe forming lens, forfocusing electrons emitted from the electron source, and a specimenstage positioned in the vacuum chamber under the probe forming lens forholding the specimen. The electron microscope further includes anintegrated x-ray detector which is positioned in the vacuum chamber, thex-ray detector including an x-ray sensitive solid-state sensor.

Structural support for positioning and supporting the detector,including the sensor, within the vacuum chamber is provided and theentirety of this support is positioned within the vacuum chamber. Nopart of the x-ray detector, other than as required for electricaloperation thereof, is located exterior to the vacuum chamber.

Regarding the second feature of the present invention, pertaining to thethermal management of the x-ray detector, an electron microscope isprovided in combination with at least one x-ray detector. The microscopeincludes a vacuum chamber containing a specimen stage for holding aspecimen to be analyzed and also including at least one x-ray detectorincorporating a solid state x-ray sensor, mounted within the vacuumchamber. The x-ray detector is integrated into the microscope wherebystructural or mechanical support for the sensor includes at least onethermo-electric cooler element for cooling the x-ray sensor. All of thethermo-electric cooler elements are retained within the vacuum chamber,none of them being positioned exterior to the vacuum chamber, and thestructural support itself provides a thermal conduit for thethermo-electrically cooled sensor(s), and the structural support isengaged with the housing structure of the microscope as a heat sink,either interior to or exterior of the vacuum chamber.

In one embodiment, the x-ray sensor and the entirety of the structuralsupport for the x-ray sensor are contained within the vacuum chamberwhereby the mechanical support is attached to the housing structureinterior of the vacuum chamber as a heat sink. In another embodimentthereof, the structural support for the x-ray sensor may exit through aport in the vacuum chamber and the structural or mechanical supportexteriorly engages the microscope housing as a heat sink.

This invention discloses a new type of EM design with integrated EDXdetection capabilities. This is a non-obvious invention in that theprovisions for supporting the x-ray sensor module(s), especially thethermal conduction requirement, must be integrated into the design ofthe EM itself, which has heretofore not been a concern of EMmanufacturers nor anticipated by EDX system manufacturers.

The subject invention will be specifically described in terms ofintegration of commercially available SDD detectors pre-mounted in acircular housing such as a TO-X case. The invention is obviously alsoapplicable to other kinds of compact X-ray detectors that can beadequately cooled by thermoelectric cooling means, or which may requireno cooling whatsoever. This includes older types of modular detectors(such as PIN diodes, which are commonly employed in hand-held x-raymonitors), as well as future generations of chip-based x-ray detectorsthat may be developed. Further, although it is certainly a convenientstarting point to utilize off-the-shelf packaged detector modules (ofthe type illustrated in FIG. 3) it is anticipated that there will befurther advantages gained by developing custom packaging of the sensorelements that is more suitable for such use than the packaged modulescurrently available. The distinction between a packaged sensor and a“bare” sensor may further be blurred by the potential development ofrobust sensors that do not require a separate vacuum window to protectthem. The integration into a SEM-type instrument of discrete sensorelements (rather than packaged modules) is thus anticipated as anobvious extension of the present invention.

A primary aspect of the subject invention is that it teaches how an EDXdetector may be beneficially integrated into the structure of the EM,rather than being a removable port-entry device as has been priorpractice. A distinguishing characteristic of several variants of thisinvention relative to prior art is that the EDX detector(s) sointegrated cannot be regarded as unitary elements that are installed viaa microscope's chamber ports(s). Another way of expressing this verysignificant aspect of the present invention is that no mechanicalcomponent of the EDX detector need penetrate the vacuum envelope of themicroscope—only the passage of electrical connections must be providedfor. Novel and improved manners of implementing such electricalconnections are also taught.

A second key aspect of the present invention is that in order toaccomplish the above integration, a novel method of thermal managementis practiced whereby structures of the electron microscope itself areemployed in the thermal circuit whereby the sensor is cooled, the term“circuit” being employed in the sense of a mechanism whereby heat istransported away from the sensor to a heat sinking mechanism.

A third aspect of the present invention is that it teaches aparticularly efficient and economical manner of accommodating multiplesensors, resulting in novel configurations of sensors that have notheretofore been practiced as a single detector system.

A fourth aspect of the present invention is that certain innovationsdeveloped in support of the first three aspects also have application inthe practice of a modular port-mounted detector of novel design.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages appear hereinafter in the followingdescription and claims. The accompanying drawings show, for the purposeof exemplification, without limiting the scope of the present inventionor the appended claims, certain practical embodiments of the presentinvention wherein:

FIG. 1 is a perspective schematic representation in vertical crosssection of a conventional Si (Li) EDX detector;

FIG. 2 is an isometric schematic illustration showing the manner inwhich a conventional Si(Li) EDX detector unit is installed on aconventional SEM or EPMA microscope;

FIG. 3 is an isometric view in partial section of a packaged SDD module;

FIG. 4 is an isometric view in partial section illustrating internalelements of a conventional SDD detector employing a packaged SDD module;

FIG. 5 is a side view in partial vertical section schematicallyillustrating the internal elements of an x-ray detector employing apackaged SDD module as incorporated in the microscope of the presentinvention;

FIGS. 6 and 7 are perspective views illustrating implementation of anintegrated EDX detector subassembly constructed with a packaged SDDmodule to be incorporated in the structure of the present invention;

FIG. 8 is a schematic illustration in partial vertical cross sectionillustrating one embodiment of the present invention wherein an array ofdetectors of different types are incorporated into the electronmicroscope;

FIG. 9 is a bottom perspective view of the circuit board for theelectron microscope of the present invention which provides a thermalbarrier interposed between the electron column and the vacuum chamber;and

FIG. 10 is a top view of the circuit board shown in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several variations of the subject invention are now described in orderto illustrate the salient features of the invention. The examples arechosen to illustrate how the key innovation—incorporation of the x-raysensor into the structure of the microscope—facilitates a number ofuseful variations that can be achieved in conjunction with additionalinnovative elements. Not all of the innovative elements are employed ineach of the illustrated examples.

EXAMPLE 1 Basic Single-Sensor Detector

FIG. 5 illustrates a very basic implementation of an integrated EDXdetector constructed with a packaged SDD module. The mounting bracket[25] is designed to be attached to the flat “ceiling” surface of thespecimen chamber [82] of a particular SEM by means of screw passageholes [81] provided. The detector element [26] is a packaged SDD module[20] of the type shown in FIG. 3. The dimensions of the mounting bracket[25] are chosen such that the SDD module [26] is held in the desiredproximity to the specimen with its axis pointing at the nominalimpingement point of the microscope's electron beam on the specimen. Thethreaded thermal stud [15] of the packaged SDD module [26] is screwedtightly into a tapped hole located in the front of the thermal interfacestub [27] whose rear portion is tapered and provided with a centraltapped hole. The role of the thermal interface stub is to provide anefficient thermal bridge between the packaged SDD module [26] and thecold plate [28]. The cold plate incorporates a mating tapered bore inits front face into which the tapered rear of the thermal stud isinserted. A screw [29] inserted through the rear of the cold plate [28]pulls the thermal interface stub [27] into intimate contact with thecold plate, a good thermal contact being abetted by the conical tapersof the two parts. The cold plate [28] in turn clamps a TEC module [30]tightly to the front lip of the mounting bracket [25] by means of thefour clamping screws [31].

The function of this arrangement is to efficiently extract heat from thedetector module [26] and, by means of the TEC element [30], to transferit to the mounting bracket [25], which in turn conducts it to thestructure of the specimen chamber [82], where it is distributed throughthe substantial thermal mass of the specimen chamber and dissipated byconvection from its surface and by conduction to associated structures.Since the amount of heat that is extracted from the packaged SDD module[26] is rather small, the additional heat generated by thethermoelectric circuit does not appreciably raise the temperature of thespecimen chamber, and this can be minimized, if required, by makingexternal provisions to facilitate convection and/or conduction of suchexcess heat from the microscope. Such provision may be as simple asproviding structural elements, such as fins attached to the specimenchamber, that facilitate convective cooling. In the specific microscopefor which this implementation was designed, an external forced-airsource ensures a flow of ambient air over the specimen chamber, and thisprovision alone has thus far been found to be an adequate means ofdissipating the minimal heat generated. In an extreme case, such asmight be presented by an instrument intended for operation in anespecially hot environment, a fluid-based heat exchanger orrefrigeration device could be employed to cool the specimen chamber. Thesalient point is that such provisions for cooling of the specimenchamber of a microscope designed with this requirement in mind can beaccomplished much more readily than the problem of dealing with the“spot cooling” of a conventional x-ray detector mounted on aconventional electron microscope.

Note that the specimen chamber of an electron microscope mustnecessarily be maintained under a relatively high vacuum while inoperation and thus there is essentially no convective transfer of heatbetween any of its internal components. On the one hand, this isadvantageous because it minimizes the parasitic transfer of heat to thecooled sensor device. However, the lack of air molecules to transferheat across small gaps makes it essential that there is intimatemechanical contact between the various elements of the thermal circuit.Ensuring such contact is the purpose of the tapered interface betweenthe thermal interface stub [27] and the cold plate [28]. It is alsoessential that the cold plate [28] and the mounting bracket [25] makegood thermal contact with the opposing faces of the TEC module [30].Techniques for providing a good thermal interface between surfaces arewell known in the art. Careful preparation of the surfaces to ensurethat they are free from surface irregularities is essential, but notsufficient. There exist a variety of thermal “grease” compounds as wellas compressible thermal contact pads or deformable foils which aredesigned to be used between thermal elements to bridge any remaining gapirregularities. Of course, any such material must be selected carefullyto ensure that it is compatible with vacuum usage.

The materials chosen for structures in the thermal path also play a rolein the success of this scheme. In the preferred implementationillustrated, the thermal interface stub [27] and the cold plate [28] arefabricated from copper. The mounting bracket [25] is fabricated fromaluminum. Both of these materials are notably good thermal conductors.On the other hand, the clamping screws [31] should not provide anefficient thermal path for transfer of heat across the TEC module.Stainless steel screws are an acceptable choice, due to the rather poorthermal conductivity of this material. Screws fabricated from amechanically-strong low-outgassing plastic such as PEEK or Vespel are aneven better choice, and PEEK screws are employed in the preferredimplementation.

In order for the SDD device to operate per specifications, its sensorelement must be maintained at a temperature of approximately −20° C.Thus, it is necessary to provide a temperature reduction ofapproximately 45° C. between the ambient temperature of a typicallaboratory environment (˜25° C.) and the SDD sensor element. The TECdevice [13] internal to the SDD module [20] could, in principle, providethis differential. However, the practical reality is that one cannotrely on the detector having access to this low an ambient temperature inorder to sink the heat it generates. For example, one particular EMinstrument designed for non-laboratory operation is specified forambient operating temperatures as high as 35° C., and after makingallowance for higher temperatures within the case of the instrument, theinternal ambient temperature may be in excess of 40° C. It is well knownthat the efficiency of a TEC device declines rapidly with increasingtemperature differential. For example, a specific TEC module driven by0.6 amps of current provides 2.43 watts of cooling across an 18° C.temperature differential, and only 0.25 watts of cooling across a 60° C.differential. Thus, it is impractical for a single-stage TEC module toprovide the large temperature differential that is required for an SDDdetector to be operated for best performance in a warm environment. Asillustrated in FIG. 4, the conventional approach is to employ a secondTEC element [22], mounted exterior to the microscope, which removes heatfrom a long cold finger [21] that receives the heat from the SDD module[20], and which second TEC element [22] then transfers its heat toconvective fins [23] incorporated in the case of the detector exteriorto the microscope, whereby the heat is dissipated to the ambient air.However, this approach is not ideal in that: (1) the cold fingerrepresents a substantial thermal mass that must be cooled to a suitabletemperature on start up. Further, a certain amount of parasitic heat“leakage” to the cold finger by both radiation and conduction isinevitable by virtue of the fact that the cold finger is supported inclose proximity to its tubular enclosure and that the cold finger mustpenetrate the specimen chamber of the microscope through some kind ofvacuum seal. This parasitic heat transfer acts to increase the demandsplaced on the external TEC module, requiring it to dissipate a greateramount of heat. (2) A conventional detector design has few practicaloptions for increasing the thermal dissipation from the hot side of itsexternal TEC device. Electron microscopes have not generally beendesigned with any provision for thermal management of the detectorenvironment and, due to the extreme vibration sensitivity of theseinstruments, the incorporation of fans within the EDX detector unit isstrongly discouraged. Thus, the only practical option for ensuringadequate heat dissipation in warm environments is to incorporateextra-large fins on the external detector case to enhance passiveconvective cooling. Since the detector is often mounted in a verycrowded area of the microscope, such large cooling fins are undesirable,and there is no certainty that they will receive adequate air flow inany case. By contrast, the simple design illustrated here minimizesthese issues. There is a very minimal thermal mass interposed betweenthe packaged SDD module [26] and the secondary TEC module [30], andthere is little opportunity for parasitic heat transfer. Further, to thedegree that the support bracket [25] is warmer than the specimen chamber[82], parasitic heat transfer actually aids the function of carryingheat away from the sensor. Consequently, there are some useful thermalefficiencies inherent to this type of design. It will also be readilyapparent that it is a far simpler mechanical problem to provide a goodthermal path to the specimen chamber than it is to provide an isolatedpath to an external sink while penetrating the specimen chamber wall.Lastly, it is to be noted that it is generally a simpler problem todissipate an amount of heat from the rather substantial vacuum chamberof the electron microscope than it is to control the temperature in thespecific locale of the detector. That is to say, by practicing thermalmanagement as an issue associated with the microscope design, ratherthan just the concern of the detector manufacturer, more efficient andreliable thermal performance can be realized.

The only remaining aspect required to make this simple design into afunctional detector is to provide the electrical connections needed tooperate the unit. For this implementation, this is accomplished by meansof a simple wiring harness (not shown in the figures). Miniature pinsockets are attached to the ends of the harness wires, which sockets arethen pressed onto the pins [19] of the packaged SDD module [20] or [26].The other end of the wiring harness is terminated by a multi-pinreceptacle which mates to a vacuum-sealed electrical feedthru thatpasses through the wall of the specimen chamber (commercial sources ofsuch feedthrus are known to those familiar with the construction ofvacuum systems or they may be custom fabricated). On the exterior of thespecimen chamber, the feedthru is connected to suitable detectorelectronics and to suitable controllers to operate and regulate the TECmodules (both the one [13] interior to the packaged SDD module and thesecondary one [30] that sinks the heat from the packaged SDD module).These electronic components may be obtained from commercial suppliers orthey may be custom-fabricated according to well-known principles.

It will be apparent that the extreme simplicity of the mechanicalcomponents of the detector here illustrated presents a striking contrastto the construction of a conventional SDD-based detector unit asillustrated in FIG. 4. Most notably, the elimination of the conventionalcold finger and its precisely-manufactured vacuum-sealing coverrepresent a significant simplification. Further, it will be appreciatedthat the interior-mounted detector of the present example permitssubstantial simplification in the design of the EM itself since there isno need to provide an in-line port to allow a conventional detector tubeto be inserted into close proximity to the specimen. Rather, therelatively small electrical feedthru that is required can be located ona convenient face of the specimen chamber. Thus, integrating thedetector into the microscope's specimen chamber in this fashion affordsgreat flexibility in design. However, it is also apparent that adetector of the type illustrated here is not a “generic” device whichcan be inserted into any electron microscope. Rather, for an optimalimplementation, the detector and the microscope must be plannedtogether. One of the important considerations is that of thermalconductivity of the microscope's specimen chamber. In the implementationillustrated here, the specimen chamber is constructed of aluminum, whichprovides an efficient thermal path to conduct heat from the detector'smounting bracket.

The specific means by which the detector is mounted into the microscopeis, of course, open to many kinds of variation. The design hereillustrated provides for connection to the “ceiling” surface of thespecimen chamber, but it is a simple matter to adapt the support bracket[25] for mounting to any convenient surface that provides sufficientheat dissipation. There is, of course, nothing prohibiting theattachment of such a detector to a port cover of the microscope if thatis the most convenient mounting point. If the port is sufficientlylarge, the cover may also house the electrical feedthru and its size maypermit the detector to be inserted through the port in a manner similarto a conventional tube-mount design. This arrangement might beparticularly advantageous in the case where the specimen chamber isconstructed of a material which is not a good thermal conductor (e.g.:stainless steel). In such a case, the attachment to a copper or aluminumport plate (for example) fitted with external cooling fins could providethe necessary thermal dissipation means. But unlike the conventionaltube-mount configuration, there is no need for the thermal path tophysically penetrate the specimen chamber or be thermally isolated fromit, and the port need not be located in a line-of-sight orientationrelative to the specimen. Thus, port mounting of an x-ray detectorconstructed according to the present teaching is an option that maysometimes be used to advantage without departing from the spirit of thedisclosed invention nor sacrificing its virtues.

It will also be apparent that, with appropriate design, the removal fromthe microscope for servicing of a detector constructed according to thisdisclosure can be made quite simple. In the present example, it involvesremoval of several screws and unplugging the electrical harness cablefrom the feedthru connection. Thus, although the design of the EDXdetector is integrated into the structure of the electron microscope, itmay still retain desirable modularity in terms of installation andservicing.

The simple detector illustrated here provides a low-cost yet effectivemeans for providing EDX capability in an electron microscope such as aSEM or EPMA. However, the functionality of this design can be readilyenhanced by simple modifications that will be apparent to one withordinary skill. For example, it might be deemed desirable to enclose thedetector in a simple housing for both cosmetic and protective reasons.Also, it is a simple matter to alter the design to accommodate differentsizes or styles of x-ray sensor modules (including the incorporation ofan electron trap for UTW types), or to alter the location andorientation of the SDD module to achieve desired variations in thegeometry. It can further be noted that even this very simple detectordesign is conducive to the installation of multiple EDX detectors withina microscope, since multiple correctly-oriented ports need not beprovided. And lastly, it can be noted that it would be a simple matterto migrate elements of the exterior electronic support circuitry intothe specimen chamber within the context of this design (in the manner ofthe next example). In short, once the tyranny of the conventionalline-of-sight, thermally-isolated, modular tube-mount x-ray detectorconvention is abandoned in favor of a design employing an internalsecond-stage TEC with a thermally-integrated support structure, manyuseful options and simplifications become available to the designer.

EXAMPLE 2 A Column-Integrated Array Detector

The prior example illustrated the practical benefits that can beachieved with a very simple application of certain of the principlesherein taught. This second example illustrates a more sophisticatedimplementation that incorporates additional novel practices and providesadditional benefits.

It will have become apparent that an important key to accomplishing anefficient integration of a packaged SDD module into an electronmicroscope is in achieving a compact thermally-efficient coupling of thepackaged SDD module to the secondary TEC module. FIG. 6 illustrates sucha mounting. Here the thermal stud of the packaged SDD module [26] isscrewed into a tapped hole in the stem of a “tee-shaped” copper stub[32] whose flat face is in contact with the cold face of the second TECmodule [30]. A clamping plate [33] with a central opening is employed toclamp the warm face of the second TEC module [30] against a flatthermally-dissipative surface (not shown) by means of four clampingscrews [31].

The subassembly depicted in FIG. 6 can be employed as the basiccomponent of a variety of different detector configurations. It has thevirtue of providing a very simple, yet rotationally-adjustable, couplingbetween the TEC module [30] and the packaged SDD module [26]. Not onlydoes this coupling represent a very minimal thermal mass, but it alsominimizes the number of thermal joints. A subtlety of this design isthat the distance of insertion of the sensor can be customized byadjusting the length of the stem of the tee-shaped mounting stud [32]and the centering of the detector in the module can be adjusted byaltering the location of the center hole in the thermal clamp plate[33]. Thus, this simple mount can be easily adapted for optimization ofdifferent detector sizes and configurations.

Rather than attaching wires to the pins from the packaged SDD module aswas done in the prior example, FIG. 7 illustrates how this can be moreconveniently achieved by means of a small printed circuit board [34]provided with pin sockets [35]. The circuit board may in turn beprovided with a small connector [36] permitting attachment of a signalcable via a mating plug. Further, the circuit board provides aconvenient place to install electronic circuitry in support of thepackaged SDD module. In particular, it is advantageous to implement thefirst high-gain stage of the preamplifier and circuitry regulating thebiasing of the sensor FET on this circuit board in order to minimizelead lengths and the opportunity for noise pickup. The packagedcharge-sensitive amplifier device [37] shown in the figure is acommercially-available component representative of the kind of circuitrythat may reside in proximity to the SDD module per this design. Thechoice of the particular circuitry chosen to reside on this board is anaspect of the art of preamplifier design which is independent of theteachings of the present invention, but it will be apparent to thoseskilled in the art that the opportunity to incorporate substantialcircuitry in such close proximity to the SDD module, within thewell-shielded environment of the microscope vacuum chamber, can bebeneficial to the critical objective of a low-noise preamplifier design.It will also be apparent that implementation of such an advantageousarrangement is conventionally precluded by the requirement to fit thesensor into a tube of minimal diameter.

FIG. 8 illustrates the design of a column-integrated array detector thatincorporates sub-assemblies [38, 38′, 38″], three differently arrangedin FIG. 9 and four in FIG. 10. The illustrated design permits up to fourof these subassemblies to be closely arrayed about the focusing lens[39] of a particular SEM instrument designed for this purpose. Thefocusing lens [39] is sealed by means of o-ring [40] and secured to acolumn mounting flange [41], which is in turn removably attached to adetector mounting interface flange [42], which provides theappropriately-angled thermal mounting face for the TEC element [30] ofmultiple detector subassemblies [38, 38′, 38″]. Sandwiched between thelens mounting flange [41] and the detector mounting interface flange[42] is a printed circuit board [43], that is sealed to said flanges bymeans of the pair of opposed co-radial o-rings [44] and said circuitboard is also mechanically secured to the detector mounting interfaceflange [42] by means of screws (not shown). This circuit board [43] thusresides both interior to and external to the vacuum enclosure and servesa number of beneficial functions, as will be shortly explained. Thedetector mounting interface flange [42] is secured to the vacuum chamber[45] of the electron microscope by screws (not shown), with substantialthermal contact between said elements, and with vacuum sealing providedby an o-ring [46]. Also supported by and electrically connected to thecircuit board [43] is an annular BSE detector [47] that residesimmediately under the probe-forming lens [39]. One SDD detectorsubassembly [38] is fitted with an SDD sensor module of UTW type with 10mm² active area and is equipped with an electron trap [48] constructedaccording to known principles of magnetic deflection. Detectorsubassembly [38′] incorporates a PIN diode type sensor, and detectorsubassembly [38″] incorporates a 30 mm² BeW type sensor module. Thus,this example illustrates the manner in which x-ray detectors ofdifferent configurations and types may be conveniently integrated into acommon array, as well as other types of detectors, such as the BSEdetector [47].

It will be observed that the detector array consisting of the detectormounting interface flange [42], the circuit board [43], the detectorsub-assemblies [38, 38′, 38″], the electron trap [48], and the BSEdetector [47] constitute a modular assembly that can be demounted fromboth the column mounting flange [41] and the specimen chamber [45], asshown in FIGS. 9 and 10. It will be readily appreciated that suchmodular attachment is of great benefit for both manufacture and serviceof the detector array since all elements of the array can be installedand their alignment observed and adjusted separately from the remainderof the microscope.

The circuit board [43] serves a number of beneficial functions:

-   -   1. It provides a convenient means for interior electrical        connections to the detectors constituting the array, employing        the kinds of electrical connectors and wire harnesses commonly        used with printed circuit boards.    -   2. It provides a convenient, compact, and economical means of        transmitting electrical power and signals through the vacuum        interface without relying on vacuum feedthrus, or requiring        microscope ports for installation of said feedthrus.    -   3. It provides a thermal barrier between the column mounting        flange [41] and the detector mounting interface flange [42].        Since magnetic focusing lenses [39] as are commonly employed in        electron microscopes generate substantial amounts of heat, it is        desirable to isolate this heat source from the detector mounting        interface flange [42] and the specimen chamber [45] which        together serve to dissipate the heat produced by the detector        subassemblies [38, 38′, 38″].    -   4. It provides the ready means for implementing support        circuitry within the vacuum enclosure.    -   5. On the exterior of the microscope, it provides a convenient        place to install additional circuitry and/or attachments to        associated support electronics, employing conventional circuit        board components.

It will thus be observed that the use of a common circuit board whichspans both the interior and the exterior of the vacuum envelope, thoughnot indispensable to the implementation of an integrated array detectoras herein taught, is both a novel and particularly beneficial practicein its own right. Though the unusual physical arrangement of such acircuit board as a means of providing electrical connection between theinternal and external elements of a vacuum system is previously known invacuum system practice and has precedent in the context of electronmicroscope construction, it is not known to have been previouslypracticed in the context of electron microscope signal detectors as istaught here, nor is the thermal isolation aspect previously practiced.It will be easily appreciated that this novel arrangement for makingelectrical connections through the vacuum envelope would be impracticalwithin the conventional art of modular tube-mounted EDX detectors. Itwill also be appreciated that the use of this arrangement would not bepractical in the context of most conventional microscopes where theelectron optics are not, as is the case here, designed to be readilydemountable. Thus, despite the many benefits accruing, this novelpractice would not be an obvious application of known art.

It will be apparent to those familiar with the art of vacuum systemsthat the materials used to construct the circuit board [43] as well asthe small circuit boards [34] and the elements interiorly attachedthereto must be compatible with the level of vacuum attained in thespecimen chamber. For the levels of vacuum necessary for satisfactoryoperation of a relatively “low vacuum” instrument, this issatisfactorily accomplished using conventional materials. In the case ofa “high vacuum” implementation, special circuit board materials arecommercially available if needed.

It will be apparent that the array of detectors here illustrated canaccommodate from one to four EDX detectors. Because of the intrinsicallysimple nature of this arrangement, as well as the ability to sharecommon resources (such as power sources) this arrangement issubstantially less costly than installing multiple EDX detectors ofconventional modular tube-mount configuration. Since an array ofsuitable mounting ports in the microscope is not required, as would bethe case for conventional port-mounted detectors, the microscope designis also simplified. Great flexibility also accrues to the fact thatdetector geometries can be readily altered by variations in the detectormounting interface flange [42] without altering the specimen chamber[45].

Another important area of novelty associated with the present exampleconcerns the provision for integrating diverse detectors in a commonarray. Conventional practice is to treat each detector as a discreteunit, separate from other detectors and separate from the microscope.Multi-element-sensor detectors known in the prior art have utilizedidentical sensor elements. In addition to teaching the practice ofintegrating EDX detectors into the structure of the microscope, as hasbeen illustrated here, the subject invention also teaches the practiceof creating a subassembly of the microscope which incorporates multipledetectors of diverse types. This practice takes two significant forms:

-   -   1. The invention teaches the creation of an integrated detector        array that can readily incorporate different varieties of        solid-state x-ray sensors and thus be usefully configured to        achieve specific analytical objectives where differing detector        varieties are advantageous.    -   2. It will be appreciated by those with skill in the art that        the concept of a demountable array onto which detectors can be        installed greatly facilitates the incorporation into an electron        microscope of many different detector types, in addition to        x-ray sensors. The common mounting structure facilitates optimal        location and adjustment, as compared to independent port-mounted        devices, as has heretofore been the practice. This facilitates        performance improvements through more optimal packing of the        sensors as well as obvious economies in construction. Again, it        will be noted that this desirable result has not been feasible        in the context of conventional microscope design practice, and        thus has not been heretofore pursued. Though the current example        specifically teaches the incorporation of a back-scattered        electron (BSE) detector into an array of EDX sensors of the SDD        type, the utility of this innovation may productively be        extended to the incorporation of other kinds of sensors and        “analytical tools” such as Raman probes, cathodoluminescence        detectors, secondary electron detectors, mechanical probes, and        the like.

This example again illustrates the practices of thermal managementtaught by this invention. It will be noted that the detector mountinginterface flange [42] is constructed of aluminum in the preferredimplementation and is in substantial thermal contact with the remainderof the specimen chamber [45], such that the whole constitutes aneffective means of dissipating heat generated by the TEC modules used tocool the detectors. This particular implementation is particularlyefficient in that the detector mounting interface flange [42] functionsas part of the vacuum envelope of the specimen chamber [45], and beingthus exposed to ambient air, it serves the dual role of thermaltransmission and thermal dissipation. The latter role might easily beenhanced, as desired, through incorporation of deliberately convectivestructures affixed to the exterior of this interface component.

Finally, it will be appreciated that though the examples here providedcouple a single TEC element to each SDD sensor element, that this is nota requirement of the practices taught in the subject invention. Designsutilizing perhaps larger TEC modules that provide cooling for multiplesensors mounted on a common thermal substrate are also anticipated.

1. An electron microscope comprising: a vacuum chamber for containing aspecimen to be analyzed; an optics column, including an electron sourceand a final probe forming lens, for focusing electrons emitted from saidelectron source; a specimen stage positioned in said vacuum chamberunder said probe forming lens for holding the specimen; an x-raydetector positioned within said vacuum chamber, said x-ray detectorincluding an x-ray sensitive solid-state sensor and a mechanical supportsystem for supporting and positioning said detector, including saidsensor, within said vacuum chamber; characterized in that the entiretyof said mechanical support system is contained within said vacuumchamber.
 2. The electron microscope of claim 1 wherein no part of saidx-ray detector, other than electrical components required for operationof said detector, is located exterior to said vacuum chamber.
 3. Theelectron microscope of claim 1 wherein said vacuum chamber includesmeans interior of said vacuum chamber for attachment of said supportsystem.
 4. The electron microscope of claim 1 wherein said supportsystem includes a portion of said vacuum chamber.
 5. The electronmicroscope of claim 4 wherein said support system includes a structuralinterface between said optics column and said vacuum chamber.
 6. Theelectron microscope of claim 1 including a FET associated with saidsolid-state x-ray sensor and additional circuitry to support operationof said solid-state sensor and associated FET, wherein said additionalcircuitry is contained within said vacuum chamber.
 7. The electronmicroscope of claim 6 wherein said additional circuitry includesamplification circuitry for acting upon electrical output of said FET.8. The electron microscope of claim 1 wherein said solid-state x-raysensor is a silicon drift detector (SDD).
 9. The electron microscope ofclaim 1 wherein a plurality of x-ray sensors are mounted within saidvacuum chamber on said support system.
 10. The electron microscope ofclaim 1 including an additional sensor, of a type other than asolid-state x-ray sensor, said additional sensor also attached withinsaid vacuum chamber to said support system.
 11. The electron microscopeof claim 10 wherein said additional sensor is a backscattered electrondetector.
 12. The electron microscope of claim 1 including an electroniccircuit board, wherein said electronic circuit board is sealed betweentwo components of said vacuum chamber by means of seals applied to theopposite faces of said circuit board such that an area of the circuitboard interior to said seals is within said vacuum chamber, and an areaof the circuit board exterior to said seals is exterior to said vacuumchamber, said circuit board having conductors for carrying electricalsignals between components of said x-ray detector located both exteriorand interior to said vacuum chamber.
 13. The electron microscope ofclaim 12 including an additional sensor which is supported within saidvacuum chamber, and wherein electrical signals associated with saidadditional sensor are also carried by conductors in said circuit board.14. The electron microscope of claim 13 wherein said additional sensoris a backscattered electron detector.
 15. The electron microscope ofclaim 1 including a thermal circuit for removing heat from saidsolid-state x-ray sensor.
 16. The electron microscope of claim 15wherein said vacuum chamber is incorporated in said thermal circuit. 17.The electron microscope of claim 16 wherein a thermal barrier isinterposed between said optics column and said vacuum chamber.
 18. Theelectron microscope of claim 17 wherein said thermal barrier iscomprised of an electronic circuit board for carrying electrical signalsassociated with the operation of said x-ray detector.
 19. The electronmicroscope of claim 18, including an additional sensor supported withinsaid vacuum chamber, wherein electrical signals associated with saidadditional sensor are also carried by said circuit board.
 20. Theelectron microscope of claim 16 wherein said x-ray sensor is a silicondrift detector (SDD) and said thermal circuit includes at least onethermoelectric cooling element.
 21. In combination, an electronmicroscope and an x-ray detector, said microscope including a vacuumchamber containing a specimen stage for holding a specimen to beanalyzed, and said x-ray detector including a solid state x-ray sensormounted within said vacuum chamber, and a thermal circuit engaged withsaid sensor and which incorporates at least one thermoelectric coolerelement for purposes of removing heat from said sensor; the improvementcomprising said thermal circuit incorporating said vacuum chamber as ameans of dissipating output heat from said thermoelectric coolingelement.
 22. The combination of claim 21, including a mechanical supportfor said x-ray detector including said x-ray sensor, wherein saidsupport functions as an element of said thermal circuit for conductingheat to said vacuum chamber.
 23. The combination of claim 22 whereinsaid mechanical support is entirely contained within said vacuumchamber.
 24. The combination of claim 21, wherein said x-ray detectorincludes multiple x-ray sensors sharing said thermal circuit.
 25. Thecombination of claim 21 including a thermal insulating barrier thermallyisolating said vacuum chamber from optical elements of said electronmicroscope.
 26. The combination of claim 25, wherein said thermalinsulating barrier is comprised of a circuit board retaining electroniccircuitry for said x-ray detector.
 27. The combination of claim 26, saidcircuit board including conductors for carrying signals from said x-raydetector within said vacuum chamber to the exterior of said vacuumchamber.
 28. The combination of claim 21, including a series of saidthermoelectric cooler elements.
 29. The combination of claim 21, whereinsaid solid state x-ray detector includes a SDD sensor.
 30. Incombination, an electron microscope and an x-ray detector, saidmicroscope including a vacuum chamber containing a specimen stage forholding a specimen to be analyzed, and said x-ray detector including asolid state x-ray sensor mounted within said vacuum chamber and athermal circuit which incorporates a multiplicity of thermoelectriccooler elements for purposes of removing heat from said sensor; whereinall said thermoelectric cooler elements are contained within said vacuumchamber.